1. Field of the Invention
The present invention relates to an endoscope system including a light source device that supplies driving currents to light emitting elements having a plurality of colors and causes the light emitting elements to emit lights.
2. Description of the Related Art
As a light source device for illumination light to be radiated from an endoscope to a subject, a xenon lamp or the like has been used conventionally. However, according to spread of LEDs, a light source device including light emitting elements such as LEDs has been put to practical use taking into account low power consumption and durability.
When white light is formed using LEDs, three colors produced by a red LED, a green LED, and a blue LED are enough for forming the white light. However, to also enable narrowband light observation (narrow band imaging: NBI (registered trademark)) that can highlight and observe a blood vessel, a configuration further including a violet LED (V-LED) has been proposed. In the configuration including the violet LED, operation is performed to cause both of the blue LED and the violet LED emit light for blue among red, green, and blue, which are three primary colors forming white light, and give a margin to light intensity of blue light.
A light source device (a frame-sequential system) having such a configuration including the red LED (R-LED), the green LED (G-LED), the blue LED (B-LED), and the violet LED (V-LED) as the light emitting elements is specifically configured, for example, as shown in FIG. 1 related to the present invention.
FIG. 10 is a timing chart of conventional LED lighting control at a time when normal observation is performed with a maximum light amount. FIG. 11 is a diagram showing spectra at a time when the conventional LED lighting control shown in FIG. 10 is performed.
One frame is equally divided into three fields. One field is divided into an exposure period and a readout period. When a light amount is maximized, an LED corresponding to the field is lit at all points in time during the exposure period. Pulse width modulation (PWM) is not performed. At this point, a maximum rated current Igmax is supplied to the green LED as an electric current (see FIG. 10). The red LED, the blue LED, and the violet LED are respectively set to current values for enabling a color balance with respect to an emitted light amount of the green LED. That is, current values less than maximum rated currents Irmax, Ibmax, and Ivmax are supplied to the LEDs of the other colors having higher light emission efficiency than the green LED. An example in which relative light emission intensities of lights emitted from the respective color LEDs are shown as spectra is FIG. 11.
Light emission luminance (brightness) in such a light source device can be adjusted by, for example, increasing or reducing driving current values supplied to the LEDs of the respective colors or performing the pulse width modulation described above.
Concerning color balance adjustment of a light source device, for example, in paragraph [0084] of Japanese Patent Application Laid-Open Publication No. 2011-36361, it is mentioned that, as a method of changing a color tone without changing emitted light amounts of respective light sources, the respective light sources are individually lit and exposure times of an image pickup device 21 at respective lighting timings are changed to simulatively change a color tone of illumination light. More specifically, the respective light sources and the image pickup device are simultaneously controlled, respective exposure times, i.e., an exposure time of the image pickup device for lighting only a first light source such as a blue laser light source to pick up an image and an exposure time for lighting only a second light source such as a white light source are respectively individually increased or reduced to be adjusted, and obtained respective picked-up images are combined as observed image data. Therefore, the color tone change in the publication is considered to be performed by a so-called element shutter.
Further, as a technique for changing brightness while keeping a color balance, for example, in paragraphs [0020], [0025], and [0027] and FIG. 2 of Japanese Patent Application Laid-Open Publication No. 2010-158415, it is mentioned that a light source section 22 includes a first LED 22a for red light emission, a second LED 22b for green light emission, and a third LED 22c for blue light emission and adjustment of driving amounts (e.g., current values in a case of current driving or duty ratios in a case of pulse driving) of first to third LEDs 22a to 22c is performed by a light receiving section 26 that measures emitted light amounts of the first to third LEDs 22a to 22c and an LED driver 21 and that a reference-voltage changing section 21a calculates first to third reference voltage values V1 to V3 according to an instruction signal related to an emitted light amount setting value set by a user such that a ratio of intensities of red emitted by the first LED 22a, green emitted by the second LED 22b, and blue emitted by the third LED 22c is fixed.
On the other hand, concerning adjustment of a light source device involved in aged deterioration of LEDs, for example, in paragraph [0050] of Japanese Patent Application Laid-Open Publication No. 2010-158415 described above, it is mentioned that lights from the first to third LEDs 22a to 22c are received by the light receiving section 26 to acquire information concerning the emitted light amount and the driving amounts and the emitted light amounts of the first to third LEDs 22a to 22c are accurately adjusted taking into account fluctuation in the emitted light amounts due to the aged deterioration, a temperature change, and the like.